1. Field of the Invention
The present invention relates in general to an upper support used in a suspension system of a motor vehicle, for elastically connecting a body of the vehicle and a shock absorber of the vehicle, and more particularly to such an upper support which has improved vibration-isolating capability while assuring high steering stability of the vehicle.
2. Discussion of the Prior Art
In a conventional suspension system of a motor vehicle, a generally cylindrical upper support is usually interposed between a body of the vehicle and a piston rod of a shock absorber which is linked with an axle or arm for supporting vehicle wheels. Such an upper support is adapted to prevent input vibrations received from the wheels through the shock absorber from being transmitted to the vehicle body, for example. The upper support generally consists of a cylindrical inner rigid member to which the piston rod of the shock absorber is fixed, a cylindrical outer rigid member which is disposed radially outwardly of the inner rigid member and fixed to the vehicle body, and an elastic body interposed between the inner and outer rigid members for elastically connecting the two members.
The upper support as described above is required to be given a soft spring characteristic for improved vibration-isolating capability, for the purpose of preventing transmission of the vibrations from the shock absorber toward the vehicle body. At the same time, the upper support is required to exhibit a high degree of stiffness so as to minimize the amount of deformation thereof with respect to a static load applied thereto, for the purposes of avoiding changes in the posture of the vehicle while the vehicle is turning, braked, or rapidly accelerated or decelerated, and thus assuring high steering stability of the vehicle.
To meet the above requirements, the upper support utilizes its dynamic spring or elastic characteristic to enhance the vibration-isolating capability, and its static spring or elastic characteristic to enhance the steering stability of the vehicle.
However, it is extremely difficult for the known upper support to fully satisfy these requirements for the vibration-isolating capability and the steering stability, since the known upper support relies only on the elastic deformation of the elastic body for isolating the input vibrations. Namely, since there is a predetermined correlation between the dynamic and static spring constant characteristics of the elastic body, it is impossible for the upper support to be given a soft dynamic spring characteristic, without exhibiting a soft static spring characteristic. Further, since the upper support in particular is likely to receive a relatively large static load during use, the elastic body must be made of a rubber material having a relatively high static spring constant, which inevitably causes stiffening of the dynamic spring characteristic of the upper support. It is also to be noted that the dynamic spring constant of the elastic body tends to increase with an increase in the frequency of the input dynamic vibrations, while at the same time the upper support should function to insulate relatively high-frequency vibrations, such as road-oriented noises, which have frequencies of several tens or hundreds of herz. Accordingly, the known upper support suffers from an extremely high dynamic spring constant when it is subjected to the middle- to high-frequency vibrations.